1. Field of the Invention
The present invention relates to a pyrogenically prepared silicon dioxide (fumed silica) powder having a low thickening effect, a process for preparing the pyrogenic silicon dioxide powder, and a dispersion comprising the pyrogenic silicon dioxide powder.
2. Discussion of the Background
DE-A-10145162 describes a hydrophilic, pyrogenic silicon dioxide in the form of aggregated primary particles having an average particle size of less than 100 nm which preferably has a specific surface area of 25-500 m2/g (measured by the BET method in accordance with DIN 66131 and 66132) with a minimal surface roughness. The pyrogenic silicon dioxide further preferably has a fractal surface dimension of less than or equal to 2.3 and/or a fractal mass dimension of less than or equal to 2.8.
DE-A-102004063762 describes a hydrophilic, pyrogenic silicon dioxide in the form of aggregated primary particles having an average particle size of 0.5-1000 nm which preferably has a specific surface area of 1-1000 m2/g (measured by the BET method in accordance with DIN 66131 and 66132). The pyrogenic silicon dioxide additionally preferably has a fractal surface dimension of less than or equal to 2.3 and/or a fractal mass dimension of less than or equal to 2.8.
DE-A-102005007753 describes a hydrophilic, pyrogenic silicon dioxide in the form of aggregated primary particles having an average particle size of 0.5-1000 nm which preferably has a specific surface area of more than 10 m2/g, preferably 75-350 m2/g (measured by the BET method in accordance with DIN 66131 and 66132). The pyrogenic silicon dioxide additionally preferably has a fractal surface dimension of less than or equal to 2.3 and/or a fractal mass dimension of less than or equal to 2.8.
The hydrophilic, pyrogenic silicon dioxides described in DE-A-10145162, DE-A-102004063762 and DE-A-102005007753 are said to exhibit an increased thickening action as compared to other conventional silicon dioxides.
Liquids useful for determining the thickening action of the pyrogenic silicon dioxide include oligomeric or polymeric resins in organic solvents.
The surface roughness of the pyrogenic silicon dioxide can be quantified by means of the theory of fractals. Fractals are structures which are similar on different length scales. Many properties of fractals can be described using power laws. When, for example, the increase in mass (m) with radius (r) is examined, a mass-fractal dimension (dm) of 3 is found for the limiting case of a compact sphere. For a structure which possesses cavities and pores, the resulting mass-fractal dimension (dm) is less than 3.
Surfaces can also possess fractal properties. Here, the size of the surface goes up with the radius. For a perfectly smooth surface, the surface-related dimension is 5.
For fumed silicas, the mass-fractal dimension (dm) is generally in the range of 1.5-2.0 and the surface-related dimensions are approximately 2.0.
DE-A-19756840 describes a pyrogenic silicon dioxide which has a BET surface area of 30-150 m2/g and a fractal BET dimension, determined by N2 adsorption in the pressure range p/p0=0.5 to 0.8, in accordance with the fractal BET theory for multilayer adsorption by the method specified by Pfeifer, P., Obert, M., Cole, M. W., Proc. R. Soc. London, A 423, 169 (1989), of less than 2.605. The pyrogenic silicon dioxide described in DE-A-19756840 can be used, for example, for polishing, with a higher erosion rate being observed, due to an increased surface roughness, as compared to other conventional silicon dioxides.
The pyrogenic silicon dioxide described in DE-A-19756840 is obtained by a flame hydrolysis process, while maintaining a hydrogen ratio gamma of less than 1 and an oxygen ratio lambda of less than 1. The ratio of the amount of hydrogen introduced, including the feed hydrogen plus the hydrogen from the raw materials, to the stoichiometric amount hydrogen required is referred to as gamma. The ratio of the amount of oxygen introduced, including the feed oxygen, to the stoichiometric amount of oxygen required is referred to as lambda.